A Very Low Obstruction Combo-Telescope

[DragosN]

Usually I don't publish projects which I don't made and tested myself, but now I have to make an exception. This projects came in my mind in a very bad financial period, and there is no sign of a change in the foreseeable future. So, I decided to make it public.

From the first time I've made an Newtonian telescope, almost 10 years ago I was looking for a solution. In time, i have seen some, but all have a problem. It works only for slow  to very slow instruments. I think that  Ed Jones ''Chiefspiegler'' telescope is the fastest. An today, fast and very fast Newtonians are more and more in use, especially for photo, and especially in Europe, where the weather is good for short periods of time.

So, my idea, because until now is just an idea, is to combine a fast Newtonian, F/D=3 to 5, with a slow refractor  F/D>10, both having the same focal length. This is a drawing of my project:

It is essentially a Newton mirror 1. with a hole inside, where a refractor group 3. is placed upside-down. The light come to the refractor group by the upper small tube, through three mirrors, 4,5 and 6, pass through the refractor group, and goes to the secondary mirror 2, of the Newtonian. In this way, the part of the primary mirror which is normally not used, is now almost complete covered by refractor group. 

In my drawing I used the following data. 

1. Primary mirror, 300 mm diameter, 1200 mm focal length, F/D=4. I've choose this data because I have a 300/1200 mm mirror, and such an instrument could be a good photo tool.

2  Secondary mirror, 100 mm small axis. So, a linear obstruction of 33%

3. An 80 mm diameter, 1200 mm focal length refractor, F/D=15.

4.5.6. Tertiary mirrors, bigger than 90 mm small axis. 4. and 5. at 22,5*, and 112,5* degrees, 6. at 45* degrees. 

This will give a theoretical linear obstruction smaller than 7%. Maybe 1 or 2% bigger than this, because of the secondary offset. Anyway, a very small number, for a fast instrument.

And now, the questions I can't fully answer.

- Could such a scope be functional? 

I think it is. Of course, best results will be made with apochromatic lens, but regarding the very long F/D ratio needed, maybe even some achromatic lens could be used for visual.

- Can such a scope be made, and at what price? 

By the industry it can be. And I think that even experienced ATM can do it. Of course is not simple. and is not cheap. An APO lens, three tertiary mirrors with good reflections properties, and a somehow complicated light pass, means some significant expenses. More, there are some technical difficulties, like aligning the refractor group (lens plus tertiary mirrors) with the primary mirror, but I consider that can be done.

- What about aberations?

My main concern is about coma, because the refractors aberations are expected to be very low at such a long F/D ratio. How will a coma corrector will affect the light passing through the refractor, What will be the influence of the circular obstruction? Also, I don't have an answer.

- Would such a scope be practical?

In my opinion, such a telescope would be the closest approximation of a general instrument. Good for visual and photo, with a smaller price and size than a APO refractor of the same diameter, and maybe with optical quality closer to that of a big APO than any other.

But to all those questions, the real answers will come in time, if enough people and enterprises will dare to build it. I would like to try myself, but as i said before, my business, portrait and caricature, is flat broke (I hate selfie      ). More than this, I had some more projects which waits for better times. As a different approach to Newtonian body, with some improvements, an equatorial fork mount, and some useful photo accessories. So, if you like my ideas (already posted some on this forum), and you can spare some money, any donation is very welcomed, at this paypal address: dragos.n.802@gmail.com 

Thank you for you attention, and I'm waiting for your feedback.  

[Sporadic Dobstronomer]

An ingenious idea, but I don't think it will work.

The light from the refracting part travels a lot further than the light from the Newtonian part so their diffraction discs will not merge as though they came from a single telescope.

What you will get is the diffraction discs from the two instruments superimposed one on top of the other -which is not going to be better than the disc from the reflecting part alone.

Sorry!

[gooseholla]

To me that looks like an awful lot of complication and lenses and mirrors to form one image and then also a large cut out of a mirror. My question would be, what in terms of visual would it offer over say a Mak or Cass, or even a normal Newtonian?

John
 

[Sporadic Dobstronomer]

To me that looks like an awful lot of complication and lenses and mirrors to form one image and then also a large cut out of a mirror. My question would be, what in terms of visual would it offer over say a Mak or Cass, or even a normal Newtonian?

John

The advantage would be that you could get giant apochromatic performance with relatively small lenses.

The snag is that to work, the optical path distances through the lens part and the mirror part would have to be matched to better than 1/8 wave which is impossible with this precise configuration and very difficult if some other arrangement were tried.

[DragosN]

Sporadic Dobstronomer, and Gooseholla, thanks for feedback. At first, that's what I am looking for, as long I can't test the idea, and not even make some simulation of it on a program like Zemax. I'm not good at this kind of programs. I will think about your opinion. and I'm waiting for others.

[Ags]

Good luck with collimating that! I'll stick with my mak :-)

[astronymonkey]

Interesting concept but im not sure how well it woukd work. The focal lengths would have to be identical to give the same magnification but with different f ratios the two systems would have conflicting exit pupil sizes at the eyepiece. It would also be very difficult to collimate given the number of reflective surfaces.

Interesting idea though.

Cheers

[JamesF]

I think personally I'd give in to the temptation to get a bigger primary mirror

James

[Soupy]

Would there not be issues with image flip and inversion at first glance I am trying to work out how the image flips and inverts between each light path and I am not sure if they will match up once brought together... Anyone any wiser on this?

[DragosN]

Ags, Mak is one of my favorite scopes. But two years ago I've sold my SW 150/1800 Mak to buy a Celestron C8. I still don't know if it is better or not.

Astronymonkey, yes all that problems are real.

JamesF. I don't even dare to say that it wouldn't be less expensive, simpler, and so on, with a bigger mirror. But that not the point. The point is to look for new solution. Could be better, or not, the quest is the real thing.

And, by the way, despite of that problems, I hope that some day I will be able to built such a scope, even in a small format, to put it to the test of reality, whatever the result will be. 

Soupy, the pozition of the image is designated by that circle with a tail. I've made some test before, with a small 112 mm mirror, a 40 mm refractor lens, and 4 star diagonals of 1,25'', separate for each optical pass, refractor and reflector, to test if is correct.

[Peter Drew]

A potential flaw that came to my mind on first reading is that refractor objectives are configured to receive parallel light rays. Your design would present a converging light cone and the refractor objective would work like a focal reducer. I've often pondered this dual instrument dilemma and like you, feel there is a solution to be uncovered. The nearest probably is a Cassegrain/Newtonian with interchangeable secondaries which I have made in the past.      

[Ajohn]

I think the achromat has been shown the wrong way round Peter. Despite that it will be virtually impossible to align the images from the refractor and the newtonian, or even keep them aligned for that matter.  The focal lengths would also have to be EXACTLY the same other wise the image scale would be different. Neglecting these problems light intensity would be increased.

Dall came up with some form of low obstruction newtonian but I've never found any details. I understand that the obstruction was so low it was near refractor standards as far as results were concerned.

John

-

[DragosN]

Light to the refractor come from the back, the pink arrowed lines.

[gooseholla]

Dall came up with some form of low obstruction newtonian but I've never found any details. I understand that the obstruction was so low it was near refractor standards as far as results were concerned.

John

-

By having a small mirror about 50 feet away from the main mirror?!

John

[Peter Drew]

I've not seen Horace Dall's small obstruction design applied to a Newtonian, only to Maksutovs and Dall-Kirham Cassegrains which gave rise to the Dall-Kirkham-Dall.

Dall used an erector transfer system which picked up the primary light cone further from the primary than usual and then transferred it to the eyepiece. This typically reduced the secondary obstruction to about 15%, gave an erect image and was completely free from sky flooding even with a single spine tube. The inside of his aluminium tubed Maksutovs were unpainted!. I made a number of Maksutovs to this design and can vouch for their high contrast.      

[Ajohn]

I nearly bought one of his Maksutov designs. He was helping some one not far from B'ham to make one. Obtained the glass etc and also figured the corrector for him. Going on the size I suspect most would have fitted a 2in focuser but not Mr Dall - 1 1/4.  One of the interesting aspects was the thickness of the corrector. Designs according to Maksutov tend to have thick ones as well as that can give coma free images. It was up for sale unfinished shortly after Dall died. I thought it was too risky at the price asked and the seller wouldn't accept what I offered. The corrector would have to be coated and the mirror spot added. I also managed to get onto a university site that showed calculations on just how accurately the corrector had to be held - that was the aspect that put me off most.  Actually it also caused me to send a mac newton I bought back due to the alignment it arrived in. Russian and it had clearly been thrown together.

John

-

[astronymonkey]

How woukd a similar set up work but instead of combing images in the scope to a single eyepiece, instead the image is fed two each eye thus making a binocular but with different image sources for each eye. The brain would be able to correct for minor errors possibly.

Cheers

[michael.h.f.wilkinson]

This scope (even if you got all alignment problems sorted) would have a weird effective aperture: A circle with a hole in it like a Newton, with another solid circle by its side. Diffraction occurs at this aperture, and cannot be magicked away by combining the two later. The diffraction pattern would be weird, and very far removed from the ideal

[DragosN]

Yes, diffraction is still present, and can be more weird than at any other model of spider and obstruction. But what I was interested was an increase in contrast, but it seems there are no many chances to get this.

[Ags]

The contrast issue is a diffraction issue, but is seems your design could increase diffraction and reduce contrast.

[DragosN]

The contrast issue is a diffraction issue, but is seems your design could increase diffraction and reduce contrast.

  . Could be ...

[DragosN]

Or, it could not be. I have done some research, and it seems that the problems are not so big, or have a fix. But I will come back with specifics when I will finish.

[nytecam]

This is a very novel Newtonian design that purports to recover light lost by shadowing of the primary by secondary via a refractor of similar dimensions and identical focal length of the primary mirror fed via 'periscope'. You have the correct number mirrors [3] in the 'periscope' plus the secondary to ensure the field orientation of Newtonian and refractor match. However in other respect I suspect the design is flawed.

The secondary shadowing is 1/9th by area or 11% not 33% [linear aperture] as your figure. The refractor optical aberration including field distortion, coma etc must precisely match the Newtonian to avoid double images and eyestrain. This will be costly to achieve. Precise collimation of each plane mirror in the 'periscope' needed so the image is not rotated. Assuming the same percentage reflectivity of all 5 mirrors in the telescope, the light-loss in the 3-mirror 'periscope' will be proportionally degraded. Each optical system will cause diffraction effects that combine to degrade images. in AP the larger size of the central obstruction is not troublesome.

In practice a standard Newtonian with the highest quality coatings will largely counter any light loss through secondary shadowing. The primary mirror need not be cored out to receive the refractor and overall more cost effective and efficient. If it's any help you could minimise the shadowing with a smaller secondary using a Dall transfer lens to project the prime image out of the Newtonian.

[Sporadic Dobstronomer]

I realised today that my second comment was wrong; this design would not work as a substitute apochromat even if you found a way to equalise path lengths because to achieve apo performance the entrance leading to the refracting section would have to be in the middle and not off to the side.
 

My point about path difference still stands: light passing through the two sections must travel exactly the same total distance if the diffraction patterns are to merge. I notice that someone at "Overcast Evenings" pointed out the same thing.

The unhappy conclusion is that this design, for all its ingenuity, has no advantages in resolution or light gathering compared with just having a Newtonian that is very slightly bigger.

The one thing it could potentially do is to have an image a few percent brighter when used at the extreme of low magnification and large exit pupil but the difference would be almost impossible to achieve or to notice if you did achieve it .

Sorry I can't bring better news.

[gooseholla]

Perhaps someone should produce a way for adaptive optics to be installed cheaply into an amateur telescope. That would bring far better results than a dual scope system surely?